> Is there a way to conditionally use proxy_ignore_headers? I'm trying to
> only ignore headers for requests which have $skip_cache = 0 for example

If you want different proxy_ignore_headers settings for
different requests, you have to use different location{} blocks
for these requests. You can do so either by using distinct
path-based locations, or by conditionally routing some requests to
a different location (e.g., with the "rewrite" directive).

In the particular case of requests to /abc, consider something
like this:

Note well that it makes little to no sense to only ignore Expires
and Cache-Control on cached requests, since these headers are only
used by nginx for caching. If caching is not used, these headers
are ignored anyway. See http://nginx.org/r/proxy_ignore_headers
for details.